The Magnetic Field within the Earth

• Published in: Proceedings of the Royal Society of London. Series A, Mathematical and physical sciences Mathematical and physical sciences, 1949-07, Vol.197 (1051), p.433-453
• Main Author: Bullard, Edward Crisp
• Format: Article
• Language: English
• Subjects: Angular velocity; Convection currents; Electric current; Electric fields; Electromagnetism; Kinetics; Magnetic effects; Magnetic fields; Secular variations; Sine function
• ISSN: 1364-5021; 0080-4630; 1471-2946; 2053-9169
• DOI: 10.1098/rspa.1949.0074

The paper discusses the magnetic effects of internal motions in the core of the earth. It is shown that tidal friction, fluctuations in the rate of rotation, nutation, and the variation of latitude have negligible magnetic effects. Precession is also ineffective if Poincare’s theorem on the precession of a liquid sphere in a rigid shell is applicable to the earth. Thermal convection is shown to be likely to occur in the core.The conservation of angular momentum will require it to be associated with a radial gradient of angular velocity which will have a large magnetic effect.Its interaction with the dipole field can produce a toroidal field which is many times as intense as the dipole field. The convective and rotational motions can interact with the dipole and toroidal field in away that tends to reproduce the dipole field. The complete theory has not been worked out, but it seems likely that the interaction is strong enough to maintain the field.The whole process resembles that occurring in a self-exciting dynamo. The existence of a strong field in the core removes the difficulties previously found in the theory of the secular variation.